In a hybrid vehicle having an engine and a dynamo-electric machine, a technique has been known using a motor generator as a self-starting motor (starter motor) for an engine. That is, the motor generator cranks and starts the engine. The motor generator mounted in this type of vehicle has higher power in comparison with a self-starting motor, and thus is capable of cranking with relatively high rotational speed. As the rotation upon cranking of the engine increases, however, the discrimination of the self-sustaining revolution of the engine from the revolution dependent on the output from the motor generator becomes more difficult; hence, the start-up of the engine cannot be readily determined.
To solve this problem, techniques for determining the engine start-up based on both the engine speed and the operational state of the motor generator have been studied. For example, one technique measures an elapsed time since the output from the motor generator fell below a reference value during cranking, and determines that the engine has started when the engine speed after the elapse of a predetermined time (a base time) is a predetermined speed (a base speed) or higher (e.g., see Patent Literature 1; Japanese Unexamined Patent Application Publication No. 2000-186654). Another technique drives an engine with an increased torque instruction value of the motor generator, then temporarily cancels the torque assist, and determines that the engine has started if the engine speed does not drop in this state (e.g., see Patent Literature 2; Japanese Unexamined Patent Application Publication No. H8-261118). These techniques can determine the self-sustaining revolution of the engine.
According to these conventional techniques, however, the determination of the engine start-up takes time, thereby precluding proper control of the engine. For example, the technique disclosed in Patent Literature 1 (Japanese Unexamined Patent Application Publication No. 2000-186654) requires the sum of a first elapsed time from the start of cranking to a time when the output from the motor generator falls below the reference value and a second elapsed time after the output fell below the reference value. The time for determining the engine start-up thus cannot be shorter than the sum of the elapsed times. The same can also be applied to the technique disclosed in Patent Literature 2 (Japanese Unexamined Patent Application Publication No. H8-261118). That is, the time for the determination of the engine start-up depends on the setting time until the torque assist is temporarily canceled.
Another potential technique determines that the engine has started based on an elapsed time after the start of fuel injection, without confirmation of the engine speed or the operational state of the motor generator. This technique, however, may erroneously determine the engine start-up even if the engine is not spontaneously revolving, and cannot improve the accuracy of the determination.
Thus, these conventional techniques barely achieve both a reduction in the time for engine start-up determination and an improvement in the accuracy of the determination at the same time.